The invention relates to, and is based on, an arrangement according to the precharacterising clause of the main claim.
Arrangements of this type are known (U.S. Pat. Nos. 4,873,484 and 4,943,764). The large dynamic range is divided up between two or more diode rectifiers, the output voltage of each individual diode rectifier being evaluated only when it is operating in a usable section of its quadratic characteristic-response range. The output voltages of the diode rectifiers are fed via a measurement-range changeover switch to a single evaluation circuit, i.e. the output voltage of only a single diode rectifier is ever evaluated. If, in the event of abrupt changes in the measurement quantity or owing to modulation, the usable characteristic-response section of the currently active diode rectifier is departed from, this makes it necessary to terminate the measurement taking place, switch over to a favourable diode rectifier, i.e. one which is operating in the usable characteristic-response range, wait for the transient settling time of the evaluation circuit to elapse, and start a new measurement. This makes it difficult or impossible, in particular, to observe one-off (transient) processes which are not covered by the dynamic range of a single diode rectifier. Especially in the case of radiofrequency signals with a time-varying envelope-curve power, in which the ratio between the maximum value and the average value of the envelope-curve power is very large, such known methods which operate with range changeover have the disadvantage that the measurement range must be selected in such a way that individual brief power peaks do not overdrive the circuit. The effect of this, however, is that the circuit is driven at a low level for most of the time, so that the signal-to-noise ratio is small.
It is an object of the invention to provide such an arrangement for measuring the rms value of a voltage over a large dynamic range, in which these disadvantages are avoided and which permits accurate error-free measurement even in the event of a time-varying envelope-curve power, for example in response to abrupt changes in the measurement quantity or owing to modulation.
This object is achieved, on the basis of an arrangement according to the precharacterising clause of the main claim, by the characterising features thereof. Advantageous refinements are given in the dependent claims.
In the arrangement according to the invention, a changeover switch is obviated and, for each diode rectifier, a separate channel is provided to treat the voltage components measured by the diode rectifier. In this case, the term xe2x80x9cvoltage componentxe2x80x9d is intended to mean either a fraction or a multiple of the input voltage to be measured. Separate analogue/digital conversion with subsequent digital signal processing takes place in each channel. When very fast A/D converters are used, the digitising of the separate channels could also take place sequentially, i.e. in these cases it is sufficient to have only a single A/D converter, the processing path being split again into two separate branches immediately after this A/D converter. The digital values of all the channels are correspondingly weighted according to their information content and are scaled. The sum of the weighting factors is in this case one. The individual weighting factors, or their ratio, is derived from the drive level of at least one of the diode rectifiers. The measurement channel for the higher powers will, for example, deliver measurement results at the lower limit of its measurement range which have greater measurement errors than in the more sensitive measurement channel. With increasing power, the situation will be reversed, for example owing to overdrive effects in the more sensitive measurement channel. Depending on the relationship between measurement error and power, different weighting functions will therefore be used in the context of the dependent claims. So that the results from the individual channels can subsequently be added to give an output value, the weighted digital values of the individual channels furthermore need to be scaled in such a way that they can be represented as a fraction or as a multiple of a common quantisation unit.
An arrangement according to the invention is suitable not only for the accurate measurement of radiofrequency signals with a constant envelope-curve power as a function of time (CW signals) but also, above all, for the accurate measurement of radiofrequency signals with a time-varying envelope-curve power, as is the case for example with modulated radiofrequency signals. This is because, in the arrangement according to the invention, a measurement value is obtained, for each individual sample value of the A/D converter, by weighting and scaling the corresponding digital value.
An arrangement according to the invention for measuring the rms value of an AC voltage over a large dynamic range is suitable not only for measuring the electrical power of, for example, radiofrequency signals, but also for measuring the rms value, or root mean square value, of any other physical quantities over a large dynamic range, which are converted into a voltage by appropriate transducers, as is the case for example when measuring mechanical stresses by a measurement transducer or the like. The arrangement according to the invention can also be used in the same way in this case.
The invention will be explained in more detail below with the aid of schematic drawings with reference to an exemplary embodiment.
FIG. 1 shows the block diagram of a circuit according to the invention for measuring the electrical power over a large dynamic range.